The present invention relates to an optical unit, a backlight device, a liquid crystal module and a liquid crystal display apparatus, with improvements in luminance uniformity.
Liquid crystal display apparatuses have been advanced with a larger and thinner display screen. A larger display screen for such display apparatuses, however, requires improvements in luminance uniformity that is degraded over a large screen which is referred to as luminance non-uniformity, hereinafter.
One of the major factors in the screen luminance non-uniformity is non-uniform luminance distribution of light of a backlight device, one of the components of a liquid crystal display apparatus, that illuminates a liquid crystal display panel.
An improved backlight device for a liquid crystal display apparatus with reduced luminance non-uniformity in a large display screen is described, for example, in Japanese Un-examined Patent Publication No. 2008-003243 (referred to as Citation 1, hereinafter).
The backlight device described in Citation 1 is installed in the back of a liquid crystal display panel, equipped with a light source, a light diffusing plate, a first light diffusing sheet, a light collecting sheet, a second light diffusing sheet, aligned in this order.
A unit constituted by the light diffusing plate, the first light diffusing sheet, the light collecting sheet, and the second light diffusing sheet is referred to as an optical unit, hereinafter.
Light beams emitted from the light sources are diffused by the light diffusing plate and the first light diffusing sheet and then collected by the light collecting sheet while being diffracted in a direction orthogonal to the light collecting sheet. The collected light beams are then emitted from the light collecting sheet, with enhanced luminance in a certain range of view angle. The light emitted from the light collecting is incident on the second light diffusing sheet to undergo diffusion to have higher luminance uniformity.
A thinner liquid crystal display apparatus requires a shorter distance (referred to as optical-unit distance, hereinafter) between the light source and the light diffusing plate, when it employs the backlight device of Citation 1. A typical optical-unit distance is about 15 mm in known liquid crystal display apparatuses.
An optical-unit distance shorter than 15 mm, however, causes a bigger difference in luminance between the section just above (in front of) the light source and the other sections with respect to the light diffusing plate. Such a difference in luminance results in luminance non-uniformity of illuminating light emitted from the backlight device.
One solution to such a problem is providing additional several optical sheets to the optical unit, which, however, causes cost-up and decrease in luminance.
Another solution to such a problem is providing more light sources while maintaining sufficient luminance. Providing more light sources, however, leads to increase in circuit components in a driver for the liquid crystal display panel, such as an inverter (not shown), which results in cost-up and higher power consumption.
The above two solutions improve luminance uniformity for illuminating light emitted in a direction orthogonal to the backlight device whereas do not improve luminance uniformity sufficiently for illuminating light emitted in the other directions.
More optical sheets and/or light sources are required to improve luminance uniformity for illuminating light emitted in the other directions discussed above, which are, however, not practical solutions.